1. Field of the Invention
The present invention relates to an image projection apparatus such as a liquid crystal projector, and particularly to an image projection apparatus having a projection lens shifting function and a temperature focus correcting function.
2. Description of the Related Art
Image projection apparatuses introduce light from a light source to a projection lens (projection optical system) through a light modulating element such as a liquid crystal panel or a digital micro-mirror device, and project an image onto a projection surface such as a screen through the projection lens.
In such image projection apparatuses, variation in temperature thereof often generates focus variation of the projection lens, which causes the apparatus to project an out-of-focus image. Thus, some image projection apparatuses are provided with a temperature focus correction function to correct the focus variation of the projection lens due to the temperature variation. Japanese Patent Laid-Open No. 2007-241260 discloses an image projection apparatus that controls a position of a focus lens included in a projection lens depending on a temperature detected by a temperature sensor to perform temperature focus correction of the projection lens.
Moreover, Japanese Patent Laid-Open No. 05-027324 discloses an image projection apparatus that has a lens shift mechanism to move (shift) a projection lens in a direction orthogonal to its optical axis with respect to a light modulating element so as to adjust a projection position of an image on a projection surface.
However, such lens shift changes an area (light passing area) of the projection lens through which light from the light modulating element passes, and therefore focus variation amounts (that is, out-of-focus amounts of projected images) for a same temperature variation amount are mutually different.
FIG. 9 shows (A) a situation in which a projection lens 105 is located at a shift position where light from a light modulating element 102 passes through a central area of the projection lens 105 to reach a projection surface 401, and (B) a situation in which the projection lens 105 is located at another shift position where the light from the light modulating element 102 passes through a marginal area of the projection lens 105 to reach the projection surface 401. In FIG. 9, reference numeral 402 denotes a lens shift amount, and reference numeral 403 denotes a shift amount of the projected image on the projection surface 401. Reference numeral 101 denotes a light source lamp.
In the two situations (A) and (B) shown in FIG. 9, since optical path lengths from the projection lens 105 to the projection surface 401 are different from each other, the out-of-focus amounts of the projected images on the projection surface 401 are mutually different even if the temperature variation amounts are equal to each other.
The temperature focus correction performed in the apparatus disclosed in Japanese Patent Laid-Open No. 2007-241260 cannot reduce such out-of-focus amounts corresponding to the shift positions (shift amounts) of the projection lens only by itself.